The present invention relates to an improved mass flow controller apparatus which is capable of controlling the mass flow rates of a gas through a conduit to be substantially constant at very low mass flow rates, and very low gas pressures.
A conventional mass flow controller is an electronic device which operates on an adjustable aperature principle and is designed to deliver a fluid such as a gas or liquid through a conduit at a preselected mass flow rate.
The primary components of a typical commercial mass flow controller include a mass flow meter which continuously senses the actual flow rate of a gas and generates an electrical signal proportional to the mass flow rate; a command signal generator which provides an electrical signal proportional to a preselected mass flow rate; a comparator controller which compares the flow meter signal with the command signal, takes the difference between the two signals and uses the difference, if any, to actuate an electrically responsive valve, the valve also constituting a component of the flow controller.
In conventional mass flow meters, heat is applied to a sensing tube conducting the fluid to be measured or is directly applied to a fluid and the temperature of the fluid is measured after the addition. When the upstream temperature is equal to the unheated stream temperature, mass flow can be measured as inversely proportional to the temperature difference for a constant heat addition. A conventional bridge circuit can be used to obtain an electrical signal versus flow function. In another arrangement, heat is applied to a sensing tube and the temperature of the tube measured before and after the heat addition. The upstream temperature of the fluid is influenced by the heating of the tube and is nearly equal to the heater temperature at zero flow. The mass flow of the fluid is proportional to the temperature differential for a constant heat addition. In a third arrangement, heat is applied to a very small wire, probe or thermistor in the fluid stream and the cooling effect of the fluid stream is measured. Cooling of the element is a function of the mass flow. In still another arrangement, heat is applied uniformly to a tube by resistance heating and the cooling effect of the fluid measured with thermocouples to determine mass flow. In yet another arrangement, a pair of temperature sensitive resistance wire coils are wound around the outer surface of a sensing tube through which the fluid flows. The coils are heated and the rate of mass flow of the fluid, which is directly proportional to the temperature differential of the coils, is measured by a bridge circuit.
All of the above arrangements to some extent rely on the generation of a temperature differential induced by a change in the flow rate, e.g. by differential cooling or heating of the fluid, as an indication of the mass flow rate of the fluid. This necessarily requires that the fluid be thermally conductive.
The last sensing arrangement described above, i.e. using a pair of heated sensor coils, is described in Blair, U.S. Pat. No. 3,938,384, and provides increased sensor efficiency due to reduced loss of heat in the sensor elements under conventional operating conditions. In an alternative embodiment disclosed in this patent at FIG. 2 a flow splitting technique is employed to permit measurement of high flow rates. The flow splitting technique employs a bypass tube, which diverts a very small percentage of the fluid in the main tube to flow therein. In accordance with conventional flow splitting techniques, as described in more detail in U.S. Pat. No. 3,851,526, the bypass tube is typically a very thin tubular conduit which is much longer than its diameter to assure laminar flow therein. Consequently, it is not surprising that the bypass tube constituting the sensing conduit of FIG. 2 in the Blair patent is of capillary dimensions (i.e. 0.014 inches at a length of 3.0 inches). The diameter of the main flow conduit 52 of FIG. 2 is not disclosed nor is the diameter of sensing conduit 14 of FIG. 1 therein. Blair also discloses the use of an insulator material of low thermal conductivity, form-fitted around the sensing elements of the sensing conduit. The insulator material reduces the attitude sensitivity of the flow meter. In a further alternative embodiment, Blair suggests, at col. 5, the use of either two constant current sources to replace the bridge resistors of the flow meter bridge circuit, or the use of two bridge resistors of very high resistance relative to the resistance of the sensing elements coupled with a single constant current source in series with the flow meter voltage source, to reduce the influence of ambient temperature changes on the accuracy of the flow meter.
A conventional electrically operated valve is a thermal valve disclosed in U.S. Pat. No. 3,650,505 the disclosure of which is herein incorporated by reference. The thermal valve utilizes heat, generated by an electrical input signal to expand an actuator relative to a reference member. The actuator and reference member are interconnected to one end, and one or the other carries a valve head on its opposite end whereby a differential in expansion caused by a heating element moves the valve head from its seat. Thus the command signal generates heat which is used to open the valve by thermoexpansion. Representative gaseous flow rates typically handled by such thermal valve as disclosed in this patent range from 50 to 3,000 cc/min.
The command signal generator, and comparator controller are all conventional devices.
In commonly assigned U.S. patent application Ser. No. 416,164 filed on an even date herewith by the inventors herein, the disclosure of which is herein incorporated by reference, a method and an analytical device is described for determining the amount of a gas absorbed or desorbed from a solid sample. To conduct this method successfully, the capability is required of continuously controlling the mass flow rate of the gas being administered to, or withdrawn from, the sample to be substantially constant (as hereinafter defined) over extended periods of time of from about 4 to about 20 hours. This method further requires that the mass flow rate be capable of being controlled to be substantially constant when operating over a partial pressure (P/P.sub.s) range of the gas of at least between 0.02 to 1, and at very low flow rates of between about 0.2 to about 0.4 ml/min at standard temperature and pressure conditions. The partial pressure of a gas, sometimes referred to as relative pressure, is the pressure (P) of a gas at a given set of temperature and volume conditions divided by the pressure (P.sub.s) of the gas at the same conditions at which liquefaction or saturation thereof occurs. Thus, at a partial pressure of about 0.04 the gas exists in a state very near a complete vacuum. Unfortunately, it has been found that conventional mass flow controllers such as those employing the above described components are not capable of sustaining, over extended periods of time, a substantially constant mass flow rate over the entire range of mass flow rates and partial pressures described above. The need therefore arose to develop a device which possessed these capabilities, and the present invention was developed in response to this need.